The construction of a transformer is prescribed by IEC (International Electrotechnical Communication) standards Pub. 60950, etc. Namely, these standards provide that at least three insulating layers be formed between primary and secondary windings (an enamel film which covers a conductor of a winding is not authorized as an insulating layer) or that the thickness of an insulating layer be 0.4 mm or more. The standards also provide that the creepage distance between the primary and secondary windings, which varies depending on applied voltage, be 5 mm or more, that the transformer withstands a voltage of 3,000 V, applied between the primary and secondary sides, for one minute or more, and the like.
According to such standards, as a currently prevailing transformer, a construction illustrated in a cross-section view of FIG. 2 has been adopted. Referring to FIG. 2, an enameled primary winding 4 is wound around a bobbin 2 on a ferrite core 1 in a manner such that insulating barriers 3 for securing the creepage distance are arranged individually on the opposite sides of the peripheral surface of the bobbin. An insulating tape 5 is wound for at least three turns on the primary winding 4, additional insulating barriers 3 for securing the creepage distance are arranged on the insulating tape, and an enameled secondary winding 6 is then wound around the insulating tape.
In recent years, however, a transformer having a structure that includes neither an insulating barrier 3 nor an insulating tape layer 5, as shown in FIG. 1, has been used instead of the transformer having the sectional structure shown in FIG. 2. The transformer shown in FIG. 1 has advantages in that the overall size thereof can be reduced compared to the transformer having the structure shown in FIG. 2 and that an operation of winding the insulating tape can be omitted.
In manufacturing the transformer shown in FIG. 1, it is necessary, in consideration of the aforesaid IEC standards, that at least three insulating layers 4b (6b), 4c (6c), and 4d (6d) are formed on the outer peripheral surface on one or both of conductors 4a (6a) of the primary winding 4 and the secondary winding 6.
As such a winding, there is known a structure in which an insulating tape is first wound around a conductor to form a first insulating layer thereon, and is further wound to form second and third insulating layers in succession, so as to form three insulating layers that are separable from one another. In addition, there is known a winding structure in which fluororesin in place of an insulating tape is successively extrusion-coated around a conductor to form three insulating layers in all (see, for example, Patent Literature 1).
In the above-mentioned case of winding an insulating tape, however, because winding the tape is an unavoidable operation, the efficiency of production is extremely low, and thus the cost of the electrical wire is conspicuously increased.
In addition, in the case of extruding fluororesin, there is an advantage in that the insulating layers have good heat resistance, because they are formed of fluororesin. However, there are problems in that, because of the high cost of the resin and the property that when it is pulled at a high shearing speed, the external appearance is deteriorated, it is difficult to increase the production speed, and the cost of the electric wire is increased as in the case of winding the insulating tape.
In attempts to solve such problems, a multilayer insulated electric wire is put to practical use and is manufactured by extruding a modified polyester resin, the crystallization of which has been controlled to inhibit a decrease in the molecular weight thereof, around a conductor to form first and second insulating layers, and polyamide resin extruded around the second insulating layer to form a third insulating layer (see, for example, Patent Literatures 2 and 3).
However, when the polyamide resin is extruded to form a third insulating layer, the surface of the electric wire formed by the polyamide resin is soft. Therefore, there is an issue of concern about sensitivity for scratches. Since it is used for electrical and electric parts, the ends of parts are scratched, which may cause failure. There is a need for a multilayer insulated electric wire which has sufficient elongation characteristics as an electric wire and the wire surface resistant to scratches.